Support 'buffer' encoding

Since Node.js v10.0.0 `crypto.DEFAULT_ENCODING` is deprecated and the default value is now 'buffer', which makes methods default to Buffer objects. I am running into an issue where `Buffer.from` is called on a string, but the encoding is `buffer`. That throws an exception because `Buffer.isEncoding` doesn't support the `buffer` encoding.

I'd suggest to add this to allow users to integrate this with a more up-to-date version of node.

Would it already work if we just adjust `Buffer.isEncoding` and add `buffer` as one of the supported types? Or do we need to do more to make this work?